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Full text of "Text Book Of Mechanical Engineering"

Appendix VL

1071

Pp. j$j and 83*7. Stress-strain Diagrams :
Mechanical Hysteresis and Fatigue.—It has been suggested
that the phenomenon of mechanical hysteresis is an explanation
of the fatigue of materials under variation of stress, and of the
slow accumulation of permanent set. Under this theory the
diagram A, Fig. 953, would indicate what might be expected to
occur. From previous statements at p. 837, it will be seen that
on the first and each subsequent stressing and unstressing the
ascending and descending curves of stress-strain are mutually
concave, and when the base line is reached there is a horizontal

. 953.

difference L between the curves that may be termed lag. The
said theory supposes that if a sufficient interval of rest be not
allowed for the lag (being most probably a heat effect) to dis-
appear before a new increase of stress is applied, the lags would
be cumulative, and after a sufficient though perhaps an exceed-
ingly large number of repetitions the* material would reach the
plastic stage, and a permanent set would ensue, resulting in final
rupture.

Experimenting to prove the truth or otherwise of this theory,
the Author has found that it is not born$ out in practice ; but that
the diagram B, Fig. 953, shews the actual result. The first stressing
gives an elastic line o^ numbered i, and the first unstressing
traces the curve 2. Allowing no time for rest, the next stressing